The present invention relates to a focusing state determination device for a taking lens, and in particular to a focusing state determination device for a taking lens which is applicable to focus determination in autofocus control for a taking lens.
Autofocus for a video camera or the like is generally based on a contrast method. In this contrast method, high frequency components of video signals in a certain range (focusing area) among video signals (luminance signals) obtained by imaging elements are integrated to obtain a focus evaluation value. Then, focus adjustment is performed automatically such that the focus evaluation value becomes the maximum. Consequently, best focusing (just focus) at which sharpness of an image (contrast of the image) captured by the imaging elements becomes the maximum is obtained.
Japanese Patent Application Publication No. 55-76312 and Japanese Patent Publication No. 7-60211 disclose methods of determining focusing states (front focus, rear focus, and just focus) of a taking lens using a plurality of imaging elements having different optical path lengths. For example, two focusing state determination imaging elements for capturing images of an identical viewing area are arranged with respect to an imaging element for capturing images for video (video imaging element) at a position where the optical path length is longer than that of the video imaging element and at a position where the optical path length is shorter than that of the video imaging element, respectively. Then, focus evaluation values for image capturing surfaces of the focusing state determination imaging elements are found in the same manner as described above on the basis of high frequency components of image signals obtained by these focusing state determination imaging elements, and the focus evaluation values are compared. Consequently, it is determined, from a relation of magnitude of the focus evaluation values, in which state of front focus, rear focus, and just focus a focusing state on the image capturing surface of the video imaging element is. Such a determination method of a focusing state can be applied to focus determination or the like for autofocus.
In the focusing state determination in the contrast method as described above, it is known that, if luminance signals are saturated when a high luminance subject is shot, since accurate focus evaluation values cannot be obtained, a focusing state is determined erroneously to cause malfunction of autofocus. In particular, in the case where a focusing state of a taking lens is determined using imaging elements other than a video imaging element as described above, since it becomes possible to use an ND filter for only the video imaging element, it becomes more likely that luminance signals obtained by the focusing state determination imaging elements are saturated in compensation for that advantage. Thus, it is required to provide saturation preventing means which prevents saturation from occurring in video signals of the focusing state determination imaging elements.
The present invention has been devised in view of such circumstances, and it is an object of the present invention to provide a focusing state determination device for a taking lens for determining a focusing state using a plurality of focusing state determination imaging elements different from video imaging elements, wherein the device can prevent erroneous determination of a focusing state at the time of shooting of a high luminance subject.
In order to attain the above-described object, a focusing state determination device for a taking lens according to the present invention is characterized by comprising: a plurality of focusing state determination imaging elements which are provided separately from video imaging elements for capturing video images and arranged at positions where optical path lengths with respect to the taking lens are different from each other, and capture subject light entering the taking lens; and a luminance signal saturation preventing device which adjusts gains of luminance signals obtained from the plurality of focusing state determination imaging elements independently from gains of luminance signals obtained from the video imaging elements and prevents the luminance signals obtained from the plurality of focusing state determination imaging elements from being saturated, wherein a focusing state of the taking lens is determined according to high frequency components of the luminance signals obtained from the plurality of focusing state determination imaging elements.
A focusing state determination device for a taking lens according to the present invention is characterized by comprising: a plurality of focusing state determination imaging elements which are provided separately from video imaging elements for capturing video images and arranged at positions where optical path lengths with respect to the taking lens are different from each other, and capture subject light entering the taking lens; and a luminance signal saturation preventing device which adjusts charge accumulation times of the plurality of focusing state determination imaging elements independently from charge accumulation times of the video imaging elements and prevents the luminance signals obtained from the plurality of focusing state determination imaging elements from being saturated, wherein a focusing state of the taking lens is determined according to high frequency components of the luminance signals obtained from the plurality of focusing state determination imaging elements.
Preferably, when a saturated part is determined in the luminance signals obtained from the focusing state determination imaging elements, the focusing state of the taking lens is determined according to high frequency components of the luminance signals other than the saturated part.
The focusing state determination device for the taking lens can be applied to focus determination in autofocus control for the taking lens.
According to the present invention, gains of luminance signals obtained from the plurality of focusing state determination imaging elements or charge accumulation times of the plurality of focusing state determination imaging elements are adjusted independently from the video imaging elements to prevent luminance signals from being saturated. Thus, even in the case where a high luminance subject is shot, erroneous determination of a focusing state can be prevented, and malfunction of autofocus in the case where focusing state determination in the present invention is used for control of the autofocus can be prevented.